1. Field of the Invention
The present invention relates to an available charging/discharging current calculation method and a power supply device that can estimate the maximum available charging/discharging currents of batteries, and in particular to an available charging/discharging current calculation method and a power supply device that can estimate the maximum current of batteries that drive an electric motor for driving a vehicle, for example.
2. Description of the Related Art
A battery has the electrical property of deteriorating If the battery is discharged at a large amount of current when the remaining capacity of the battery is low, or if the battery is charged at a large amount of current when the remaining capacity of the battery is high. For this reason, it is important for batteries of a power supply device that drives an electric motor of a vehicle, for example, to restrict the maximum charging/discharging currents in accordance with the remaining capacities of the batteries. In the case where the maximum current is not restricted, when the remaining capacity of the battery is low, if the batteries are discharged at a large amount of current so that the voltages of the batteries abruptly fall under hard acceleration, or when the remaining capacity of the battery is high, if the batteries are charged at a large amount of current so that the voltages of the batteries abruptly increase under hard braking, problems will arise such as remarkable battery property reduction, and the like. Since in particular vehicle power supply devices charge/discharge batteries at a very large amount of current, the life of the battery will be very short if the maximum current is not restricted. This maximum current restriction is important for power supply devices used for applications that require high output of batteries such as for vehicle (e.g., automobile), bicycle and power tool.
On the other hand, a device to be driven by a power supply device requires a current as much as possible. For this reason, in order that batteries can be charged/discharged at a current as much as possible within a range in that the electrical properties of the batteries do not deteriorate, it is important to accurately calculate the maximum current that is available in the power supply device. As this type of current calculation method, a method has been known that calculates a charging/discharging current limit value by referencing a table that is previously obtained by experimental measurement based on the remaining capacities (state-of-charge: SOC) of batteries, charging/discharging current amounts, the temperatures of batteries and the like that are detected by detectors. Also, a method has been known that calculates a charging/discharging current limit value by using a predetermined model (e.g., mathematical model) based on the voltages of batteries (e.g., open circuit voltage: OCV; Vo) that are detected by detectors.
However, the table-referencing method cannot respond to the hysteresis variation of batteries. For this reason, there is a problem in that errors of the detected remaining capacity and temperatures directly affect the calculation.
Also, in the model-using method, the remaining capacity detection is significantly affected by an error of the resistance caused by a synchronization time lag between current detection and voltage detection. In addition, this method may allow an excess amount of current depending on the hysteresis states of the batteries.
When the remaining capacity is improperly detected, if an excess amount of current is allowed, batteries may have an overload in charging/discharging operation. This may cause remarkable deterioration of battery life. Also, the remaining capacity varies due to battery self discharging.
These factors cause difficulty of estimation of battery remaining capacity. For this reason, it is very difficult to accurately calculate the remaining capacity (see Japanese Patent Laid-Open Publication No. 2006-197727).
The present invention is aimed at solving the problems. It is a main object of the present invention to provide an available charging/discharging current calculation method and a power supply device that can accurately estimate the maximum available charging/discharging current values of batteries based on the charging/discharging currents, voltages and temperatures of the batteries.